After-burner chamber with secure ignition

ABSTRACT

A turbojet having an after-burner chamber presenting a specific injector opening into the burner ring to provide secure ignition. The after-burner chamber has an annular fuel injection duct, an ignition spark plug, and an ignition injector installed facing the spark plug and connected to specific fuel feed means.

The invention relates to an after-burner chamber for a turbojet, and more particularly it relates to an improvement in the ignition system for said after-burner.

BACKGROUND OF THE INVENTION

After-burning is used in military aircraft to increase the thrust of the engine quickly, and consequently to increase the speed of the airplane, when flying conditions make that necessary.

U.S. Pat. No. 5,396,761 describes an after-burner chamber having a plurality of burners extending radially, one of which is fitted with an ignition system comprising a spark plug associated with an injector. The injector has orifices facing away from the spark plug, such that ignition is not ensured under all operating conditions of the jet.

U.S. Pat. No. 3,931,707 describes an after-burner chamber fitted with a burner ring having a plurality of nozzles delivering fuel upstream from a ring of stationary vanes where the fuel mixes with air entering the ring via a downstream slot. A spark plug serves to ignite the fuel. However, there is nothing to control vaporization of the fuel to ensure that the mixture will ignite under all flying conditions.

OBJECTS AND SUMMARY OF THE INVENTION

The invention applies to a similar type of combustion chamber fitted with an annular burner and a spark plug capable of generating sparks for igniting after-burning. In such a system, the end of the spark plug is placed facing a point on an annular fuel distribution duct. In that type of after-burner, depending on the altitude and the speed of the airplane when after-burning is triggered, the pressure and the temperature at which fuel is injected into the duct can be too low or too variable. It is not possible to act directly on the injection pressure in the annular duct. Consequently, ignition is not certain and sometimes after-burning is not triggered, and that is not necessarily due to the spark plugs operating poorly. The invention serves to solve this problem.

More particularly, the invention provides an after-burner chamber comprising an annular fuel injection duct and an ignition spark plug mounted in the vicinity of said annular duct, wherein an ignition injector is installed facing said spark plug and is connected to specific fuel feed means suitable for delivering said fuel under a controlled pressure independent of the feed conditions to said annular duct.

Thus, the specific ignition injector fed by a fuel source that is different from that which feeds the burner and that is used for spraying fuel for a few seconds in front of the ignition spark plug, guarantees that after-burning will ignite.

Once after-burning has started, the specific feed source for the ignition injector can be switched off until the next time after-burning is to be triggered. The spark plug issues several sparks per second until ignition occurs. The ignition period lasts for a few seconds, e.g. three seconds to ten seconds.

The injector is provided with a special nozzle that vaporizes the jet into particles that are very fine, being smaller than fifty micrometers.

In an embodiment in which the annular duct is mounted in a burner ring, said injector projects inwards from said ring, behind the spark plug.

In an after-burner chamber, it is conventional to distinguish between a hot inner primary stream in which the gas delivered by the turbine flows, and a cooler outer secondary stream that is separated from the primary stream by a shroud. When the burner ring is placed in the hot primary stream, -the injector and the spark plug are housed in a protective sheath fastened to an outer casing, commonly referred to as the heater casing and opening out into said burner ring. The protective sheath isolates the ignition system from said hot primary stream. In addition, the protective sheath advantageously includes ventilation orifices communicating with the cold secondary stream. In this way, cold air flows continuously inside the protective sheath in order to keep the ignition system at an acceptable temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other advantages thereof will appear better in the light of the following description of a turbojet having an after-burner chamber in accordance with the principles of the invention, and given purely by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a fragmentary half-section of the upstream portion of an after-burner chamber in which the burner ring is placed in the secondary stream;

FIG. 2 is a diagram on a larger scale showing the disposition of the aeromechanical injector and of the ignitor spark plug relative to the burner ring;

FIG. 3 is a perspective view of the burner ring seen looking along arrow 3 in FIG. 2 and showing the connection of the specific injector;

FIG. 4 is a perspective view looking along arrow 4 in FIG. 2 and showing the burner ring and the specific injector; and

FIG. 5 is a section view on a radial plane through the after-burner chamber showing the ignition system of the burner ring when the ring is situated in the relatively hot primary stream.

MORE DETAILED DESCRIPTION

With reference to FIGS. 1 to 4, there can be seen a portion of the after-burner chamber 11 of a turbojet 12, situated downstream from the turbine (not shown) on its axis X′X. Conventionally, the casing 14 of the after-burner chamber is connected by bolts to the end of a diffuser casing 16 also referred to as a heater casing. An inner exhaust casing 18 is connected to an exhaust cone 19 extending axially in the central portion of said diffuser casing 16.

In the space defined between said diffuser casing 16 and the casing 18 together with the cone 19, there is a “confluence” shroud 20 which separates and channels the inner primary gas stream F₁ and the outer secondary gas stream F₂. The primary stream F₁ of gas from the turbine is at high temperature while the secondary stream F₂ coming from the compressor is at relatively low temperature and enables the structural elements of the after-burner chamber to be cooled. An inner jacket 22 carried by the casing 14 defines the after-burner chamber downstream from a combustion system 25 essentially comprising a burner ring 26 and radial arms 27 extending from the burner ring towards the inside of the after-burner chamber almost as far as the axis X′X thereof.

Between them, the jacket 22 and the casing 14 define an annular cooling channel 28 extending around the after-burner chamber 11.

There are nine radial arms 27 that are regularly distributed circumferentially. Each arm is of V-shape section, with its limbs diverging rearwards and forming two rectilinear ducts for ejecting sprayed fuel, constituting a “flame-holder” structure. In its radially-outermost portion, each arm presents an aerodynamically profiled element 30 whereby it is connected to the casing 16. This element enables the secondary stream to be channeled, in particular towards the annular cooling channel 28. These arms for maintaining combustion at the center of the after-burner chamber are of conventional design and are not concerned by the invention; they are therefore not described in greater detail herein.

The burner ring 26 of the after-burner system 25 is substantially circularly symmetrical about the axis X′X; it is carried by the elements 30. The burner ring opens rearwards between the arms 27 and the internal jacket 22. The portion that is open rearwards presents a section that is V-shaped on a radial plane containing the axis X′X. The burner ring 26 houses an annular fuel injection duct 34 comprising an internal spray tube 36 pierced by a plurality of very small diameter holes 35 and extending inside a protective tube 38 (pierced by holes of relatively large diameter that are regularly spaced apart circumferentially) forming a screen against radiation in order to provide the spray tube with thermal protection. Advantageously, the holes 35 of the spray tube 36 coincide with the holes in the protective tube 38. The annular duct 34 extends over practically the entire circumference of the burner ring 26. In known manner, said burner ring is fed with fuel via a duct 40 connected to a main source of fuel (not shown).

At a point around the burner ring 26, an ignitor spark plug 42 is conventionally installed in the vicinity of the annular spray duct 34. It is slidably engaged in a sleeve 43 which has a flange 44, itself sliding in the space defined between the outer wall of the burner ring 26 and a frame 45 welded thereto about the hole through which the end of the spark plug penetrates into said burner ring.

According to an important characteristic of the invention, an ignition injector 48 is installed facing said spark plug 42 and is connected to specific fuel feed means that are adapted to deliver said fuel under a pressure that is controlled and independent of the feed conditions to said annular duct. The injector 48 is itself said to be “specific” since it is fed by a special fuel source (not shown) via its own duct 49.

The spray endpiece of the injector 48 which projects into the burner ring is slidably engaged in a sleeve 47 which includes a flange 50, itself sliding in the space defined between the wall of the burner ring (at the front thereof) and a frame 51 welded thereto about the hole through which said endpiece penetrates into the burner ring.

In this example, the specific injector 48 is of the aeromechanical type. It is provided with a nozzle for delivering a fine spray of fuel, capable of delivering particles of a size smaller than fifty micrometers. The injector is placed in such a manner that the end of the spark plug 42 (where the spark occurs) lies in the vaporization cone 54 of the injector.

FIG. 5 shows a variant in which the burner ring 26 a is disposed in the hot primary stream F₁ (which depends on the general design of the jet). In this embodiment, structural elements that are analogous to those described above are given the same numerical references. They are not described again. Because of the location of the burner ring, provision is made to protect the ignitor system (specific injector 48 a and spark plug 42 a) in a protective sheath 60 fastened to the casing 16 and communicating with the burner ring. Said sheath 60 thus passes through the secondary stream F₂.

In addition, the protective sheath has ventilation orifices 62 communicating with the cold secondary stream. Thus, the inside of the sheath continuously carries a flow of cold air. A thermal protection screen 64 is also placed between the injector and the end of the spark plug. It presents a passage 65 facing the spray orifice of the specific injector. An annular ventilation box 66 is installed inside the burner ring 26 a.

When after-burning is ignited, fuel is injected into the burner ring 26 and the arms 27, and simultaneously, although under different pressure conditions, a small quantity of additional fuel is injected by means of the specific injector 48. The sparks produced by the spark plug 42 occur in the spray-cone of the injector, thereby causing the fuel injected by the specific injector to ignite, followed by the fuel delivered along the burner ring and the arms. 

1. An after-burner chamber comprising an annular fuel injection duct and an ignition spark plug mounted in the vicinity of said annular duct, wherein an ignition injector is installed facing said spark plug and is connected to specific fuel feed means suitable for delivering said fuel under a controlled pressure independent of the feed conditions to said annular duct.
 2. An after-burner chamber according to claim 1, wherein said injector is fitted with a nozzle for spraying said fuel finely.
 3. An after-burner chamber according to claim 1, in which said annular duct is mounted in a burner ring, wherein said injector is mounted in said burner ring and opens out to the inside thereof, in the vicinity of said spark plug.
 4. An after-burner chamber according to claim 2, of the type in which said burner ring is placed in the hot primary stream of said after-burner chamber, wherein the injector and the spark plug are housed in a protective sheath fastened to an outer casing and opening out into said burner ring.
 5. An after-burner chamber according to claim 4, wherein said protective sheath has ventilation orifices communicating with the cold secondary stream of said after-burner chamber.
 6. A turbojet including an after-burner chamber according to claim
 1. 